Magnetic micrometer



July 8, 1941- w. B. ELLwooD 2,248,569

MAGNETIC MICROMETER Filed'May 2s, 1940 'HJiuidt 'Patented July 8, 1 941 MAGNETIC mcaoME'rEa Walter B.- Ellicott, New York, N. Y., assignor to Bell Telephone Laboratories, Incorporated,-New York, N. Y., a corporation of New York Application May 23. 1940, Serial No. 336,802I

-s claims. (cl. :i3-447) This invention relates-to apparatus for measuring small distances, and moreparticularly to a magnetic micrometer for measuring the thickness of relativelythin sheet materials. r

One particular problem whiclrthia invention is peculiarly adapted to solve is the measurement of the message or s relatively thin inlay of non-magnetic material as for example a small inlay of contact metal permanently attached to a magnetic contact strip. Such measurements are made' by this apparatus without in any way destroyingl or injuring the inlay. Heretofore such measurements have been made by the .long tedious process of first sectioning the v inlay and then measuring the distance@ the margin between the inlay and the magnetic strip with a microscope micrometer. Measurements by the apparatus of this invention not only does not injure the test specimen but from numerous tests it has been found thatA they may be made rapidly and with great reliability.

It is the object of this invention to provide a means for rapidly and accurately measuring small distances substantially devoid of magnetic material.

The foregoing object is attained by this inven. tion .which provides a normally balanced magnetic bridge having two magnetic paths'which is rendered unbalanced by the insertion of the distance to be measured in one of the "paths and means for measuring the degree of unbalance.

The invention may be better understood by referring to the drawing in which:

Fig. 1 is a schematic of the essential magnetic elements and associated electrical circuits of the preferred embodiment;

Fig. 2 is an enlargement of the magnetic bridge A elements of Fig. 1 where the thickness of lan inlay is to be measured;

Fig. 3 isA similar to Fig. 2 except that it dis'- closes how the thicknessof any non-magnetic material, such as paper or foil, is measured;

Fig. 4 discloses how the thickness of a sheet of magnetic material is indirectly measured by the apparatus of Fig. 1; and

Fig. 5 shows the essential elements of a preferred form of jig for holding the magnetic bridge elements of Fig. l.

Referring now to Fig. l, the magnetic bridge comprises two similar collinear probes I, 2 preferably of some good magnetic material, such as permalloy. These probes may be laminated if desired but for most practical embodiments they are of relatively small cross-sectional area and may be solid. One of these probes is preferably ilxed and the other movable and an elastic means hereinafter more particularly described is used to urge them together and to include between their engaging faces a third strip or sheet of magnetic material 3. o A

Magnetic probes I, 2 and strip or sheet 3, together with their inherent air leakage paths, constitute a magnetic bridge. A magnetomotive yforce is impressed on probes I and 2 by means of coils 5, 5 which have equal numbers of turns and are serially connected in' the same magnetic sense to a suitable source of valternating current supply through transformer T1 and potentiometer Ill. The alternating current voltage supply is connected to the primary 8 of transformer T1 via terminals I9, 20 and ballast lamp BL which serves to regulate and maintain constant the voltage output of secondary 9. The current u through coils 5, 5 is adjusted by slider II of potentiometer Ill to a predetermined value as read by milliammeter I2. Terminating resistor 1 is of such magnitude as to bring the ballast lamp BL to the operating portion of its regulating characteristic. Y

'Ihe electrical circuits so far described impress alternating magnetomotive forces on probes I, 2 and in the same direction along their common axis. Referring now to Fig. 2, the instantaneous values of these magnetomotive forces are as indicated by the arrows. Assuming for the moment that magnetic'strip 3 does not contain the non-magnetic inlay 4 and that both probes I, 2 are contiguous with its opposite surfaces, it is apparent that, there are two well-defined magnetic paths, one through probe I into strip 3, out from the end 3' of strip 3, through an air path -and back to probe I at its extremity I'. The

other path is similarly traced through probe 2, extremity 2f, the air path, into the end 3' of Y strip 3, along strip 3 and back to probe 2. If it is assumed further that these two paths contain A equal air path reluctances they will be in substantial balance so that the net flux ln strip 3 is reduced to zero. Now, if another strip 3 having an inlay I of thickness T is inserted so that the exposed inlay surface is contiguous with probe 2, Y the reluctance of the path through probe 2 is increased by an amount T/llap where ap is the crosssectional area rf probe 2 and a is the relative 'permeability of the inlay', which is a constant approximately equal to unity. This unbalances the magnetic bridge and permits a resultant flux in strip 3 equal to the difference between the flux in probe I and the flux in probe 2. The magni- Y tude of this difference in flux is, moreover, a.

function of the inserted reluctance and hence a function of the thickness oi!l the non-magnetic inlay.

Unbalance is detected by means of two seriallyconnected coils 6, 6 also of equal numbers of turns woundin the same sense and mounted on probes I and 2 in a manner similar to coils 5, 5. These two coils and resistors Ra and R4 comprise the two branches of an electric bridge. It is evident that ii' the two coils 6, 6 are linked by equa1fiuxes the two generated electromotive forces will be equal and additive. Moreover, if R4 is made equal to Re. the voltage across the primary P of transformer Ta will be zero because the bridge will be balanced. However, when the fluxes in probes I and 2 differ the generated electromotive forces in coils 6, 6 differ and an unbalance voltage will appear across primary P, which voltage is a delinite function of the difference in fluxes in probes I and 2 and consequently a definite function of the inlay thickness as previously stated.

Secondary S of transformer T2 supplies this unbalance voltage to the input of a full wave bridge-type rectifier I3, the output whereof is connected to a galvanometer 2I. This rectiiier may be of any type but for the best sensitivity it preferably comprises four enclosed electromagnetic relays I4, I4 and I5, I5 synchronously driven from the alternating current power source through copper oxide rectifiers I6, I1 and condenser IB. Opposite relays I4, I4 operate together on one half-cycle through rectifier I'I while during the next half-cycle relays I5, I operate together through rectifier I6. The form of relays herein disclosed is more particularly described in applicants copending application Serial No. 198,629 filed March ,29, 1938. Briefly..

however, they comprise a tubular glass envelope having a contact strip of rmagnetic material sealed in at each end so that the two free ends Kof the two strips are near each other butnot in contact. 'Ihe envelope is exhausted and filled with inertl gas. An operating coil is wound around the glass envelope so as to induce magnetism in the two strips when a current is passed through the coil whereby the two contacts are closed.

Fig. 3 discloses how the magnetic circuit would be made up when it is desired to measure the thickness of loose sheets of foil or other nonmagnetic material 4A. In this case, strip 3 which in Fig. 2 was, assumed to have permanently embedded therein an inlay 4 is replaced by a plate, rod, reed. sheet or disc 3A whose dimensions are at least large enough to cover the probes I and 2 and to extend substantially normal to their axis a distance about equal to their length. At this point it should be noted that these approximate relative dimensions are not the criteria for properly proportioning this bridge. The important criteria4 are that the probes I and 2 and strip 3 or 3A should be of a magnetic material which possesses a reasonably high permeability, that the two probes should be about identical in size and shape for magnetic symmetry and that all three parts should be of such dimensions as to maintain the two return air paths at relatively high reluctances. Also the air-gap reluctance between probe I and strip 3 or 3A should be negligible. A disc of relatively large diameter instead of a rod or strip for element 3 or 3A will therefor satisfy these criteria or in the case of Fig. 3 the strip 3A may be an integral part of probe I. Therefore, the term "strip in the appended claims Should be understood as including such equivalent terms as rod. disc, plate, reed or sheet. For the reasons given above in connection with Figs. 1 and 2 the unbalance voltage measured by galvanometer 2Iv is a function of the thickness T of sheet IA.

In Fig. 4 is shown a method of employing the apparatus of Fig. 1 to indirectly measure the thickness of sheets of magnetic material. In this case probes I and 2 are first separated and fixed at a predetermined distance by inserting a magnetic sheet of thickness D slightly greater in thickness than the sheet to be measured and clamping the movable probe 2 in place. The bridge is then adjusted for balance and the thick magnetic sheet removed. Sheets 3B are then inserted with one surface contiguous with the end of probe I as indicated. As before there will, in effect, be a distance T having a reluctance T/aap inserted in one magnetic path to unbalance the bridge. The thickness W.A of sheet 3B is then obtained by subtracting T from D or W=thickness of magnetic sheet.

D :distance between probes I and 2.

T :thickness of air space.

Fig. 5 shows a preferred form of jig which has been found useful for holding one form of magnetic strip in place. The specific form of strip 3 herein disclosed is as shown in Figs. 1 and 2 and has permanently embedded therein the test specimen in the form of an inlay 4. the thickness whereof is to be measured. It is obvious that since the structure of this specimen includes a strip of magnetic material, the latter may be utilized as part ofthe magnetic bridge. Inserting a new strip 3 for each specimen 4 to be tested, however, requires that vthe strips be limited to a substantially fixed position. Notch 3I' in rod 3| and spring clamp 34 attached to fixed support 34 perform this function. The position of notch 3|' is adjusted by thumbscrew 33 threaded in another fixed support 32.

Probe I is given a tight fit in non-magnetic holder 22 as shown which,l in turn, is securely held in fixed position by setscrew 25 at support 24. A similarly shaped non-magnetic holder 23 holds probe 2 and is adapted for longitudinal sliding movement in bearing support 26. A

'hand lever 28, fulcrumed at fixed block 29 engages holder 23 through a pin 30 to impart the necessary longitudinal movement. Spring 2'I urges holder 23 and consequently probe 2 toward the specimen with suiiicient force to insure substantially negligible air-gap reluctances where the probes engage the specimen. A retaining plate 26 is secured to support 28 to retain spring 21 in place. f

In order to use the jig for measuring magnetic sheets as in Fig. 4 probe 2 may be fixed in position by tightening thumbscrew 35 shown in Fig. 5, so as to clamp holder 23 stationary with respect to bearing 26.

Exciting coils 5, 5 are wound around the probes and are frictionally secured or cemented to their respective holders. Pick-up coils 6, 6 are similarly mounted. The electrical connections to these coils are as shown in Fig, 1 but have been deleted in this figure for the sake of clarity.

The bridge as above described may be used to measure thickness in eitherv one of two ways. First, it may be used as a direct indicator of the thickness where galvanoineter 2| is empiri- .induced in the two coils 6, 6 are equal.

cally calibrated to indicate directly the thickness of a test specimen in suitable units of length.

It can be shown mathematically that for the ,circuit shown in Fig. 1 the response of galvanometer 2| will obey the following law:

M- -.response of galvanometer 2|.

T=Ithickness of specimen.

K and C are constants which may be empirically determined.

- vanometer responses. For the case shown in Fig.

4 these constants may be determined by successively inserting in the bridge two different magnetic sheets of known thickness.

The bridge may also be used by the null" method wherein it is initially balanced with both probes I and 2 contiguous with opposite faces of a magnetic strip such as strip 3A. The electric bridge will then balance with resistor Rs equal to resistor R4 since the electromotivev forces The non-magnetic specimen to be measured is then inserted between probe 2 and strip 3 or 3A, as before whereupon the electric bridge may" be rebalanced by changing variable resistor R4 by an amount, ARA. The thickness T of the specimen then obeys the following law:

The constants E and F are determined in a manner similar to that previously `described for Equation 2. i

It should be noted that both of the abovedescribed methods result in determining the degree of unbalance of the magnetic bridge occasioned by the insertion of the non-magnetic specimen. It should also be observ'ed that the term non-magnetic specimen" includes an airgap as is indicated in Fig. 4 and that the distance T is always the distance between the magnetic strip 3, 3A or 3B and probe 2 which distance disposed rods of magnetic material with one end of each adjacent to the other, a strip of magnetic niaterial interposed between said adjacent ends so that its long dimension is substantially normal and contiguous to the engaging ends of the rods, means for displacing one of the rods from the strip by a distance to be measured, said distance being substantially devoid of any magnetic material, means for impressing substantially equal and varying magnetomotive forces-in each of the rbds to produce varying fluxes therein, and electric means responsive to said fluxes and adapted to indicate the condition of balance of the magnetic bridge both before and after displacement.

2. An apparatus for measuring thickness comprising two collinearly disposed rods of magnetic material, elastic meanslfor urging the adjacent ends of the rods together, means for separating said ends against the action of the elastic means. means for supporting therebetween a strip of magnetic material the long dimension whereof is substantially perpendicular to the principal axis of the rods, means for magnetically exciting each of the rods with varying magnetomotive forces of equal magnitudes, and means for indicating the difference in the total uxes generated in said two rods by said magnetomotive forces.

3. An apparatus for measuring the thickness of non-magnetic material comprising a magnetic bridge having two magnetic paths, a portion common to each path including a strip of magnetic material, each of the remainder of said paths including some magnetic. material of substantially the same shape and reluctance as 'the other, each path also having in series therewith a is substantially devoid of magnetic material.

Moreover, the noni'r'agnetic specimen may actually be classed as diamagnetic or slightly paramagnetic but in the latter case its permeability must be small compared with that of the probes I and 2 and the strip 3. It therefore necessarily follows that a calibration for measuring vone kind of material will not hold for another material if its permeability is very much different.

Actual measurements made by this apparatus indicate that it is capable of great sensitivity. For one galvanometer used the sensitivity was about 3(10)6 inches per scale division and the tests indicate that this is not the limit of sensitivity.

What is claimed is:

1. A magnetic bridge for measuring small distances comprising in ycombination two collinearly larger reluctance substantially equal to that included in the other path, means for impressing substantially equal varying magnetomotive forces in each path, means for inserting in one path apiece of non-magnetic material the thickness whereof is to be measured thereby increasing the reluctance of said path and changing the total magnetic flux induced therein, and means for measuring the magnitude of the. change in said magnetic ux.

4. An apparatus for measuring the thickness of a piece of non-magnetic material permanently attached to a strip of magnetic material com-- prising a symmetrical magnetic bridge with two magnetic paths, a portion common to both paths including said strip of magnetic material, two similar rods of magnetic material symmetrically disposed on opposite surfaces of said magnetic strip and with one end of one rod contiguous to one surface thereof and one end of the other rod separated from the other surface by the thickness of said non-magnetic material, said rods forming part of the two magnetic paths means for impressing substantially equal and varying magnetomotive forces in each of the rods to induce varyingjluxes therein, the differ ence in the magnitudewhereof is determined by the thickness Vof said magnetic material, and means for measuring the difference in flux magnitudes.

5. An apparatus for measuring the thickness of non-magnetic material comprising a magnetic bridge, two similar and symmetric magnetic meshes therefor each mesh including portions of low reluctance and at least one portion of relatively higher reluctance, means for inducing varying and equal magnetomotive forces in each mesh, means for inserting said non-magnetic material in the low reluctance portion of one of said meshes whereby a magnetic unbalance is produced, the magnitude whereof is determined netic Aunbalance whereby the thickness of said' non-magnetic material is measured.

6. An apparatus for measuring thickness comprising in combination a magnetic bridge, an electric bridge having two branches, two magnetic paths for said magnetic bridge having included as parts thereof two rods and a strip ali of magnetic material so arranged that the principal axes of the two rods are collinear and said strip and the adjacent end of one of said rods thereby correspondingly changing the reluctanceof its magnetic path, and means associated with said electric bridge adapted to measure said change in reluctance as the measure of their adjacent ends contiguous and substantially `15 the thickness of the non-magnetic material.

normal to the strip, said strip forming a magnetic path common to both ofk said two paths, means WALTER B1 EILWOOD. 

